1. Field of the Invention
The present invention relates to electrochemical cells and more particularly, to an improved electrochemical cell formed by a dual laser process and having a high output and low weight.
2. Description of the Related Art
High capacitance electrochemical capacitors (ultracapacitors, supercapacitors, pseudocapacitors) have been studied for more than 40 years, yet it has only been in the last decade that they have gained widespread attention. The interest in these systems has arisen mainly from commercial applications, such as electric or hybrid powered vehicles or power backup sources, where there is a need for energy storage systems that can hold relatively large amounts of energy yet deliver it in short pulses leading to high power output. Conventional parallel plate capacitors can deliver high power, but are limited in the amount of energy they can store. Electrochemical capacitors provide an intermediate range of power and energy between parallel plate capacitors and conventional batteries.
The need for high power pulsed energy sources is also important in the development of micro-power sources for microelectronic and microelectronic mechanical systems (MEMS). For example, a microdevice may need a burst of energy for locomotion or transmission of data. Although the maximum power requirements for pulsed operations are typically much less than one watt per square centimeter, this low output power is still more than can be delivered by a thin-film microbattery. Therefore, the development of hybrid micro-power systems that incorporate a high power supercapacitor in combination with a microbattery is essential for the future of next generation micro-devices.
The structure of an electrochemical capacitor is composed of two electrically conducting plates separated by an electrolyte. This can be arranged in either a stacked or planar structure. The planar structure is particularly useful for small (˜mm-μm) sized supercapacitors where the device is located on a flat substrate such as a microchip.
The materials that compose the electrodes of supercapacitors are typically high surface area and porous carbon or other complex metal oxide systems such as ruthenium oxide or tantalum oxide. The particular mechanism for charge storage depends on the electrode materials, however, in all cases, a high surface area is a desirable attribute and must be maintained by the processing of the device. Furthermore, for the complex metal oxides, the amount of structural water in the system plays an important role in the ability to store and transport charge.
The manufacture of thin-film metal oxide supercapacitors is difficult due to the complex materials requirements for an effective, high capacitance power source. Aspects of morphological constraints and processing temperature limitations, in addition to the presence of water in the oxide structure have made metal oxide systems incompatible with prior art vacuum techniques for thin film growth such as physical vapor deposition.
Most prior art efforts to deposit supercapacitor electrode materials have resulted in ineffective or low capacitance power sources. However, the present invention incorporates a laser direct-write process to produce a high output cell. The method allows for depositing a material onto a receiving substrate by using a source of pulsed laser energy, a receiving substrate and a target substrate. The present invention incorporates this with a dual laser method for simultaneous or sequential deposition and processing the materials into electrochemical cells.
The target substrate comprises a laser transparent support having a back surface and a front surface. The front surface has a coating that comprises a mixture of the transfer material to be deposited and a liquid material. The source of pulsed laser energy can be positioned in relation to the target substrate so that pulsed laser energy can be directed through the back surface of the target substrate and through the laser-transparent support to strike the coating at a defined location with sufficient energy to volatilize material at the location, causing the coating to desorb from the location and be lifted from the surface of the support. The receiving substrate can be positioned in a spaced relation to the target substrate so that the transfer material in the desorbed coating can be deposited at a defined location on the receiving substrate and so that the material, or decomposition products thereof, in the desorbed coating can migrate from the space between the receiving substrate and the target substrate.
This laser direct-write process allows for depositing a wide range of materials such as complex polymeric materials or complex electronic materials, with no damage to the starting material, as well as the deposition can be carried out in ambient conditions, in a computer-controlled fashion and wherein the spatial resolution of the deposited material can be as small as 1 μm.
However, this prior art technique does not allow for the simultaneous creation of a capacitor. In other words, this laser direct-write process allows for the deposition of electrode material, but the electrolyte would then need to be added externally. In other words, the electrolyte would not be added during processing. The extra steps of adding the electrolyte after the creation of the electrodes creates additional problems. First, the external addition of the electrolyte provides little control over the amount of electrolyte actually added. Second, the addition of electrolyte causes device failure as the flow of liquid material can erode the electrodes. Third, the addition of excess liquid electrolyte is unavoidable at such small dimensions and leads to packaging and encapsulation difficulties. Fourth, the laser direct-write technique deposits ‘wet’ materials that need to be dried prior to laser machining. This adds additional processing steps and may damage the electrode material.